It is well known to use Manchester encoding of binary data for example for transmission of the data. In Manchester encoding, a data `1` is represented by the two-bit word 10, and a data `0` is represented by the opposite two-bit word 01. The transmitted bit rate is thus twice the data rate. Advantages of Manchester encoding include a high signal transition density (changes between `0` and `1` bits) which facilitates clock recovery, a null d.c. component in the transmitted signal spectrum, and the ability to detect data errors as sequence violations (e.g. the two-bit words 00 and 11 represent errors rather than valid data).
In decoding Manchester encoded data, it is necessary for the decoder to be synchronized to the two-bit word boundaries. For example, a series of data `1`s is encoded as a bit sequence . . . 10101010. . . ; if the decoder is out of phase with the word boundaries, this will be incorrectly interpreted as a series of data `O`s, i.e. the bit sequence . . . 0101010. . . In the event that the decoder is in phase with the word boundaries, transmission errors may occur in the Manchester encoded bit sequence, such errors appearing to the decoder as sequence violations which can cause an erroneous phase slip. In this event the decoder operates out of phase with the word boundaries until further sequence violations produce a subsequent phase slip.
Such erroneous phase slips can lead to significant problems, especially in continuous (as distinct from packetized) digital transmission systems in which they may cause loss of frame synchronization.